Microsoft officials admit that Cashback never really took hold. From a blog post on June 4 on the Bing Community blog:

"Why are we doing this? When we originally began to offer the cashback feature, it was designed to help advertisers reach you with compelling offers, and to provide a new type of shopping experience that would change user behavior and attract a bunch of new users to Bing.

"In lots of ways, this was a great feature – we had over a thousand merchant partners delivering great offers to customers and seeing great ROI on their campaigns, and we were taking some of the advertising revenue and giving it back to customers. But after a couple of years of trying, we did not see the broad adoption that we had hoped for."

Microsoft will continue running the Cashback program until 9:00 p.m. PT on July 30 and will continue to offer Cashback offers until that time. After that, Microsoft is giving customers a year to redeem any credits they've accumulated via Cashback. Microsoft also will provide 12 months of customer support for the service, officials said via the blog post.